A liquid crystal display generally comprises a liquid crystal cell, a polarizing plate and an optical compensatory sheet (phase retarder). In a display of transmission type, two polarizing plates are placed on both sides of the liquid crystal cell, and the optical compensatory sheet is provided between the cell and one or each of the polarizing plates. On the other hand, a display of reflection type comprises a reflection board, a liquid crystal cell, one optical compensatory sheet and one polarizing plate, in this order.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates, and the electrode layer has a function of applying a voltage to the rod-like liquid crystal molecules. According to alignment of the rod-like liquid crystal molecules in the cell, various display modes have been proposed. Examples of the display modes for transmission type include TN (twisted nematic) mode, IPS (in-plane switching) mode, FLC (ferroelectric liquid crystal) mode, OCB (optically compensatory bend) mode, STN (super twisted nematic) mode and VA (vertically aligned) mode. Examples of the modes for reflection type include HAN (hybrid aligned nematic) mode.
The polarizing plate generally comprises a pair of transparent protective films and a polarizing membrane provided between them. For preparing the polarizing membrane, a polyvinyl alcohol film is soaked with aqueous solution of iodine or a dichromatic dye, and is then uniaxially stretched.
The optical compensatory sheet is generally provided in various liquid crystal displays, to prevent displayed images from undesirable coloring and to enlarge a viewing angle of the liquid crystal cell. As the optical compensatory sheet, a stretched birefringent polymer film has been conventionally used.
Recently, in place of the stretched birefringent polymer film, an optical compensatory sheet comprising a transparent support and a thereon provided optically anisotropic layer formed from liquid crystal molecules (particularly, discotic liquid crystal molecules) has been proposed. The optically anisotropic layer is formed through the steps of aligning the liquid crystal molecules and then fixing the alignment. As the liquid crystal molecules, liquid crystal molecules having polymerizable groups are generally used. For fixing the alignment, they are polymerized. The liquid crystal molecules give large birefringence and have various alignment forms, and accordingly an optical compensatory sheet obtained from the liquid crystal molecules has a specific optical character that cannot be obtained from the conventional stretched birefringent polymer film.
The optical character of the optical compensatory sheet is designed according to that of the liquid crystal cell, namely, according to display mode of the liquid crystal cell. In fact, if an optical compensatory sheet is made with liquid crystal molecules (particularly, discotic liquid crystal molecules), various optical characteristics can be realized according to the display mode of the liquid crystal cell.
Various optical compensatory sheets using discotic liquid crystal molecules have been proposed according to liquid crystal cells of various display modes. For example, the optical compensatory sheet for liquid crystal cell of TN mode is described in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620A1. The compensatory sheet for liquid crystal cell of IPS or FLC mode is described in Japanese Patent Provisional Publication No. 10(1998)-54982. The compensatory sheet for OCB or HAN mode is described in U.S. Pat. No. 5,805,253 and International Patent Application No. WO96/37804. The compensatory sheet for STN mode is described in Japanese Patent Provisional Publication No. 9(1997)-26572. The compensatory sheet for VA mode is described in Japanese Patent No. 2,866,372.
In the optical compensatory sheet comprising a transparent support and a thereon-provided optically anisotropic layer formed from liquid crystal molecules, an orientation layer for controlling the alignment of liquid crystal molecules is provided between the support and the anisotropic layer. As the transparent support, a cellulose ester film is preferably used. In preparing the optical compensatory sheet, it is necessary to fix the orientation layer (normally, made of polyvinyl alcohol) closely on the cellulose ester film (transparent support).
However, the affinity between the cellulose ester film and polyvinyl alcohol (material of the orientation layer) is so poor that the interface is easily cracked or broken, and hence the optically anisotropic layer on the orientation layer easily comes off from the film (support). Particularly when the optical compensatory sheet is cut (or punched out) to size for the display, the orientation layer (together with the optically anisotropic layer) is shocked and thereby often partly peeled from the film. At that time, fragments of the peeled orientation layer (and the peeled optically anisotropic layer) are scattered and dusted on the film, and consequently cause “undesirable brilliant points” in a displayed image. The term “undesirable brilliant points” means defects undesirably sparking on a screen of liquid crystal display. In order to avoid the undesirable brilliant points (i.e., in order to enhance the adhesion between the cellulose ester film and the orientation layer), the cellulose ester film has been immersed in an alkaline solution to saponify the surface of the film or otherwise an undercoating layer made of gelatin has been provided on the film. Japanese Patent Provisional Publication No. 8(1996)-94838 describes saponification treatments of the cellulose ester film.